This invention relates to lasers. More particularly, it is concerned with stoichiometric single crystal neodymium fluoride compounds useful as lasing media and with lasers employing such media.
There has been considerable recent interest in attempts to produce miniature lasers which utilize stoichiometric neodymium compounds as the active laser media. These efforts have been reviewed by S. R. Chinn et al. in Laser Focus, pp. 64-69, May, 1976, and by H. G. Danielmeyer in Festkorperprobleme, 15:253 (1975). U.S. Pat. Nos. 4,000,247 to Yamada et al. and 3,813,613 to Danielmeyer et al. disclose, respectively, neodymium meta- and ultraphosphate laser media and lasers based on these materials.
The laser media discussed in the prior art are, for the most part, drawn from the classes of meta- and ultraphosphates, borates, and tungstates of neodymium. These materials are all examples of stoichiometric neodymium laser media; that is, materials in which neodymium forms one component of a stoichiometric formulation rather than merely a dopant.
These stoichiometric laser materials permit the construction of miniature lasers (minilasers) because of the high concentration of active lasing sites per unit volume of the crystal. The neodymium concentration in NdP.sub.5 O.sub.14, for example, is 4.0.times.10.sup.21 ions cm.sup.-3, roughly 30 times that found in conventional neodymium-doped yttrium aluminum garnet (Nd:YAG). The high density of neodymium sites in stoichiometric laser materials causes laser pump radiation to be absorbed over short distances within the crystal, typically within 100 micrometers. This in turn allows for the use of small crystals for the lasting medium, permitting the overall dimensions of the laser to be kept to a minimum. The potential applications for such minilasers in the fields of optical communications, hand-held laser ranging devices, target designators and the like, is obvious.
To be effective as a high gain stoichiometric neodymium laser medium, a material should first possess appreciable deviation from inversion symmetry at the neodymium sites in the crystal. There is a decrease in fluorescence cross-section, and therefore gain, if the ions are situated at sites possessing inversion symmetry. Second, it is important that, while maximizing neodymium ion concentration in the material, as large a separation between adjacent lasing ion sites be maintained as possible. Self-quenching of the luminescence efficiency increases with decreasing intersite separation, in turn decreasing the laser gain.
In the stoichiometric neodymium laser materials described in the prior art, intersite separation of adjacent neodymium ions is accomplished by the incorporation of an oxygen-containing anion such as phosphate, borate, or tungstate. However, reasonably strong interactions between adjacent neodymium sites in these materials contribute to radiationless energy losses which, in some cases, may become appreciable. Thus, for example, in neodymium pentaphosphate, the luminescence lifetime decreases by a factor of almost 3 when neodymium ion concentration is increased from dilute to stoichiometric.